There has been known a technique in which an air fuel ratio sensor, a fuel addition valve, and an NOx storage reduction catalyst are arranged in an exhaust passage of an internal combustion engine in a sequential manner from an upstream side thereof to a downstream side thereof (for example, see a first patent document).
Here, fuel added from the fuel addition valve is added into an exhaust gas in a liquid or liquefied state. Then, this liquid fuel may adhere to the air fuel ratio sensor. Due to this adhesion of fuel, there will be a fear that the detection accuracy of an air fuel ratio may drop. In addition, the air fuel ratio sensor is maintained at a high temperature state by means of a heater in order to activate a sensor element, so when the fuel adheres to the air fuel sensor, it will be oxidized. For this reason, there will be a fear that the air fuel ratio sensor may be overheated.
Then, even if the air fuel ratio sensor is arranged at the upstream side of the fuel addition valve, there will be a fear that the fuel added into the exhaust gas may be caused to flow toward the upstream side thereof due to the pulsation of the exhaust gas, thus adhering to the air fuel ratio sensor. This can be said that the same will also occur in the case of an NOx sensor, etc., which serves to detect an NOx concentration in the exhaust gas. In addition, it can be said that the same will occur even in cases where urea or ammonia is added as a reducing agent.